1. Field of the Invention
The present invention relates to a treating method and apparatus utilizing a chemical reaction and, more particularly to a treating method and apparatus utilizing a chemical reaction, applicable to production of chemical industrial goods and semiconductor goods such as integrated circuits or electron devices.
2. Related Background Art
Treating methods utilizing chemical reactions have been and are currently applied to many chemical industrial goods and also to semiconductor integrated circuits.
Among the chemical reactions, those in a solution are employed in production of many industrial goods because of their peculiar properties different from those of vapor-phase chemical reactions. Among others, electrolysis reaction is the technology for supplying electric energy to an electrolytic cell to cause change in substance at the interface between the electrode and the electrolyte, thereby producing a chemical substance as an objective material. This technology is applied to production of many industrial goods, including smelting of aluminum, copper, or zinc, metal plating, production of hydrogen, oxygen, caustic soda, and chlorine, and so on.
There are recent proposals on the ELTRAN (Epitaxial Layer TRANsfer by bond and etch back porous Si) SOI substrate as a method for producing a semiconductor substrate by use of a chemical reaction in a solution in the semiconductor processes (Japanese Patent Application Laid-Open Nos. 5-102445; 5-217992; 5-217821; 5-217820.; 5-275663; 5-275329; 6-342784; 7-249749; and 7-235651).
These prior art methods utilize electrochemical reactions in solution, for example, anodization, for producing a semiconductor substrate characterized by epitaxially growing single-crystal silicon on one porous surface of a silicon single-crystal substrate. Part or all of the surface has been rendered porous by the anodization process. The method further comprises bonding a surface of this epitaxial layer to an arbitrary support surface and selectively etching the porous portion of the silicon substrate or separating the bonded substrate into the non-porous portion and an SOI substrate at the porous portion.
Since the porous silicon formed by the anodization reaction has an enormous surface area and the surface thereof is active to adsorption of an organic gas, a variety of gas sensors utilizing the property are under research (M. B-Chohrin and A. Kurex, "Absorbate effects on photoluminescence and electrical conductivity of porous silicon," Appl. Phys. Lett., vol. 64, pp. 481-483, 1994; A. Richter, "Design considerations and performance of adsorptive humidity sensor with capacitive readout," The 7th Int. Conf. Solid-State Sensors and Actuators, pp. 310-313, 1993; R. C. Anderson, R. S. Muller, and C. W. Tobias, "Investigations of porous silicon for vapor sensing," Sensors and Actuators, A21-A23 pp. 835-839, 1990; Akira Motohashi, "Change in electrical characteristics of anodized silicon/single-crystal silicon samples against water vapor," IEICE Transactions (C-II), vol. J77-C-II, pp. 213-220, 1995).
As described above, the chemical reactions in a solution are widely used in production of industrial goods, for example, in the chemical industries and the semiconductor industries. Incidentally, from the viewpoint of production of industrial goods, increasing the efficiency of chemical reaction is very important in order to raise economical efficiency of material production or resource savings. Particularly in the semiconductor industry, in terms of cost efficiency, further increase in the rate of such chemical reactions as film formation and etching, as well as in-plane uniformity of chemical reactions for large-diameter silicon wafers, is demanded.
The chemical reaction in a solution, however, involves generation of a reaction by-product, which is a phenomenon of inhibiting increase in efficiency, rate, and uniformity of chemical reactions. That is, presence in the solution of a further reaction by-product formed by the chemical reaction in addition to the objective chemical substance will inhibit progress of the chemical reaction. A typical example is a phenomenon in the electrolysis in an aqueous solution where hydrogen is normally generated at the negative electrode while oxygen is generated at the positive electrode.
Since these reactions compete with deposition of an objective metal or a discharge reaction of an inorganic and organic chemical species, they are also significant in terms of current efficiency. In addition, since they can also change the kinds of reaction products, they have extremely important effects. For example, in the production of a semiconductor substrate utilizing anodization, gas molecules as a reaction by-product inhibit increase in the rate and uniformity of the reaction for forming porous silicon.
In the conventional technology, as described in the examples of Japanese Patent Application Laid-Open No. 5-102445, the gas molecules generated over the solubility thereof in the solution will form bubbles to adhere to a silicon wafer and the like, thereby inhibiting the reaction. Therefore, the process includes a treatment such as adding alcohol as a surfactant or agitating the solution by use of an agitator. Further, as described in the patent gazette of Japanese Patent Publication No. 8-31460, there is also a method of mounting a bubble removing device in a plating apparatus to remove bubbles contained in a plating solution.
These treating methods are temporary solutions, but are not essential solutions. For example, in formation of porous silicon in the production of a semiconductor substrate, pores having diameters of about 10 nm and depths of about 20 .mu.m need to be formed with restraining fluctuation within .+-.5% as to the diameters of pores and within .+-.10% as to the depths of pores in the silicon wafer plane. In addition, this process of formation of a porous silicon layer has to be carried out in one minute, which is a standard process time in recent volume production of semiconductors. Assuming that the wafer size is 6 inches, the diameters of pores are 10 nm, the aperture ratio (area of pores/area of wafer) is 0.3, and the etching rate is 20 .mu.m/min, the rate of generation of hydrogen is 788 cc/min. This large amount of hydrogen is generated at bottoms of fine and long pores having the diameters of 10 nm and the depths of 20 .mu.m, i.e., the aspect ratio of pore of 2000. In order to form the porous silicon layer, the reaction solution has to be always supplied to the bottoms of the fine and long pores having the aspect ratio of 2000 while perfectly removing this large amount of hydrogen as the by-product. It is, however, difficult to solve the above problem by such methods as the addition of a surfactant and the use of agitator. Namely, there is no technology achieved yet for producing the industrial goods including the semiconductor substrates, by utilization of a chemical reaction in a solution, with high productivity.